Plants, microorganisms and animals produce a large variety of organic chemical compounds, some of which are used universally for growth and metabolism and others seem to play specialized roles in the life cycle of the organism (Maimone & Baran, 2007). As such, two large classes of natural products are widely recognized. Primary metabolites are those essential for live in all eukaryotic organisms, while specialized metabolites appear to give species specific advantages for occupying distinct environmental niches. The distinctive role specialized metabolites play in an organisms natural history, for example how these metabolites provide protection against microbial challenge, have also not escape attention for their possible utility in a wide range of applications. For example, many of the currently used drugs are derived or inspired from plant-derived specialized chemicals and are commonly referred to as Natural Products (Buchanan et al., 2002). Capturing the chemical and structural diversity of Natural Products has recently been identified as a major objective within the scientific community in large part because of the wide array of applications Natural Products can have and the resulting economical implications.
Terpenes and terpenoids are a large and diverse family of Natural Products with more than 55,000 having been identified (Maimone & Baran, 2007). However, based on the biosynthetic mechanisms responsible for terpenes, chemists have predicted that only a small fraction of all the possible terpene compounds have been discovered (Bouvier et al., 2005). Terpenes are derived from the five carbon isoprene unit with different combinations of the isoprene units generating different classes of the terpene products. The classification and biosynthesis of terpenoids are based on the number of five-carbon units they contain as illustrated in FIG. 1. Monoterpenes (consisting of 10 carbons), sesquiterpenes (15 carbon derivatives), and diterpenes (20 carbon derivatives), arise from the corresponding intermediates geranyl diphosphate (GPP), farnesyl diphosphate (FPP), and geranylgeranyl diphosphate (GGPP). These intermediates in turn arise by the sequential head to tail condensation of C5 units. Higher order terpenes like triterpene (30 carbons) are formed from two farnesyl units condensed head-to-head. Likewise, tetraterpenes (40 carbons) are formed from two geranylgeranyl units condensed head-to-head.
Monoterpenes are well known as the volatile essence of flowers and plants and such mixtures can account for up to 5% of plant dry weight (Buchanan et al., 2002). Menthol and camphor are common monoterpenes found in diverse plant families and whose structural complexity in terms of stereo- and regio-chemistry are emphasized in FIG. 2. Besides providing pleasing fragrances, monoterpenes have been shown to function as signal molecules in defense mechanisms against pathogens (Hick et al., 1999). Hence, monoterpenes have the commercial value as flavors, fragrances, essential oils, and as anticancer and antimicrobial drugs (Burke et al., 1997). Sesquiterpenes (C15) are also found in essential oils, and many sesquiterpenes possess antibiotic activities, prompting suggestions that they are produced by plants as a defense mechanism. Diterpenes (C20) include gibberellins (plant hormones), vitamin A, as well as pharmaceutical important metabolites such as taxol, an exceptional anticancer regent (Barkovich & Liao, 2001). Triterpenes (C30) include the brassinosteroids, phytosterols important for lipid membrane composition, and components of surface waxes, such as oleanolic aid of grapes. Squalene, the major content of shark liver oil, is a linear triterpene and common ingredient in cosmetic products (Buchanan et al., 2002), has special utility as a lubricant for high performance machinery, and is a common adjuvant in many pharmaceutical formulations (Bhilwade et al., 2010, Huang et al., 2009, Reddy & Couvreur, 2009). Tetraterpenes (C40) include carotenoid accessory pigments, like lycopene, the monocyclic gamma-carotene, and the bicyclic alpha- and beta-carotenes, which perform essential for the light reactions of photosynthesis. Longer chain terpenes, so-called polyterpenes, contain more than 8 isoprene units and include examples like ubiquinone and rubber (Buchanan et al., 2002).
There are two pathways for terpene biosynthesis in plant cells. One is the mevalonate pathway pathway (MVA) which is well established and discovered in the 1960s (Bouvier et al., 2005). The other is the mevalonate independent pathway, or more properly referred to as the methylerythritol-phosphate pathway (MEP), which was more recently discovered (Bouvier et al., 2005). The MEP pathway was first discovered in prokaryote cells, and then confirmed to exist in plant cells (Barkovich & Liao, 2001). Interestingly, plants utilize these two pathways to meet different terpene biosynthetic needs. Sesquiterpenes, sterols, triterpenes and oligoterpenes (side chain of dolichols) are synthesized in the cytosol via the MVA pathway, while monoterpenes, diterpenes, teraterpenes, and polyterpenoids are synthesized in chloroplasts via the MEP pathway using pyruvate and glyceraldehydes-3-phosphate as the primary precursors (FIG. 2).
The principal product of the mevalonate pathway is sterols, for example cholesterol in animal cells, stigmasterol and campesterol in plant cells, and ergosterol in fungi, which all play essential roles in establishing the structural integrity of membranes, establishing permeability and fluidity, and also serving as signal compounds in cellular communication (Buchanan et al., 2002). In Saccharomyces cerevisiae, only the mevalonate pathway is known to operate and no components of the MEP pathway have been found (Maury et al., 2005). FIG. 3 shows the intermediates and the related genes involved in the yeast mevalonate pathway (Maury et al., 2005). Two molecules of acetyl-CoA are condensed by acetoacetyl-CoA thiolase, which is encoded by ERG10, to synthesize acetoacetyl-CoA. A second condensation reaction between acetoacetyl-CoA and acetyl-CoA is then catalyzed by HMG-CoA synthase encoded by ERG13 to yield 3-hydroxy-3methyglutaryl-CoA (HMG-CoA).
TABLE 1Biological activities and commercial applications of typical terpenoidsCommercialClassBiologic activitiesapplicationsExamplesMonoterpenoidsSignal molecules andFlavors, fragrances,Limonene, menthol,used as defensecleaning products,camphor, linaloolmechanisms againstanticancer,pathogensantibacterial,antioxidant, essentialoil, biofuelSesquiterpenoidsAntibiotic, antitumor,Flavors, fragrances,Nootkatone,antiviral, immuno-pharmaceuticalsartemisinin, patchoulol,suppressive, and(antibacterial,nerolidol, farnesol,hormonal activities,antifungal),capsidol, farnesene,defensive agents orinsecticides, biofuelsbisabolenepheromonesDiterpenoidsHormonal activities,Anticancer agents,Gibherellins, phytol,growth regulator,feedstock fortaxol, kaurene,antitumor,industrial chemicalabietadiene, kaurenoicantimicrobial andapplicationsacid, abietic acidanti-inflammatorypropertiesTriterpenoidsMembraneBiologic markers,Sterols, hopanoids,component, steroidbiofuel, skinsqualene,hormonesmoisturizers inbotryococcene.cosmetics,immunologic adjuvantin vaccines.TetraterpenoidsAntioxidants,Food additives,Lycopene, beta-photosyntheticcolorants, antioxidantscarotenecomponents,pigments, andnutritional elements(vitamins)
HMG-CoA is reduced by HMG-CoA reductase to yield mevalonate. This reaction is catalyzed by HMG-CoA reductase, which is encoded by 2 separate loci in yeast. Both loci appear to compensate for a knockout loss of the other gene. The C5 position of mevalonate is phosphorylated by mevalonate kinase, encoded by ERG12. Then a second kinase, phosphomevalonate kinase, encoded by ERGS, catalyzes the successive phosphorylation to yield diphosphomevalonate. In the next step the diphosphomevalonate is converted into IPP (isopentenyl diphosphate) by mevalonate diphosphate decarboxylase, encoded by ERG19. IPP isomerase, encoded by IDI1 converts IPP into DMAPP (dimethylallyl diphosphate). The condensation of the C5 building blocks of IPP and DMAPP into FPP is catalyzed by FPP synthase, which is encoded by ERG20. FPP can then be used as substrate for sterol and other isoprenoid biosynthetic needs.
Recent studies have discovered that FPP is also available in yeast mitochondria, as evidenced by increasing novel sesquiterpene production three-times by targeting a sesquiterpene synthase to the mitochondria compartment compared with targeting this same enzyme to the cytosol (Farhi et al., 2011). The origin of FPP in mitochondria could be the IPP and DMAPP arising in cytosol being imported and converted in the mitochondria to FPP. Alternatively, a hypothetical leucine metabolism model for the formation of terpene in S. cerevisiae is also a possibility. The leucine catabolism pathway (MCC pathway) is known to occur in the mitochondria of other eukaryotic mammal and plant cells (Anderson et al., 1998), in mitochondria leucine metabolite to form 3-Hydroxy-3-methylglutaryl-CoA, which can be catalyzed by HMGR to produce mevalonic acid, and then produce IPP and DMAPP through MVA pathway as shown in FIG. 4 (Carrau et al., 2005). Interestingly, a yeast line engineered with a chimeric diterpene synthase targeted to the cytoplasm along with prenyltransferases streamlined for GGPP biosynthesis, yielded 2-3 times more diterpene when the expression vector also provided a leu2 auxotrophic selection marker gene. The interpretation provided by the authors was that the extra leucine produced by the auxotrophic selection marker gene provided another source for IPP via the leucine catabolic pathway (FIG. 4). (Zhou et al., 2012).
Prenyltransferases generate allylic diphosphate esters GPP, FPP, and GGPP. These compounds can undergo a variety of reactions, which include cyclization reactions catalyzed by terpene synthases, yielding diverse terpenes based on regio- and stereo-chemical constraints built into the reactions. Prenyltransferases and terpene syntahases utilize electrophilic reaction mechanisms to mediate the catalytic reactions (Ohnuma et al., 1996) and typically share a conserved aspartate-rich DDXXD motif thought important for the initial substrate binding and metal-dependent ionization step leading to the first reaction carbocation intermediates. In the prenyltranferase reactions, the allylic diphosphate ester can be ionized to form a carbocation, then condensed with a second IPP in another round of elongation.
Terpenes are a very large class of structurally diverse compounds made by organisms in all kingdoms of life. The terpenes from plants are perhaps the most extensively described as evident by well over 100,000 different terpenes reported in the literature (Buckingham, 2003). Terpenes are also widely recognized for their diverse utility and applications. For example, taxol, a diterpene widely recognized for its application as a chemotherapeutic agent, was first isolated from the bark and needles of several Taxus plant species (Wall and Wani, 1995). Likewise, Artemisinin, a sesquiterpene isolated from the plant Artemisia annua, has been developed as a key pharmacological agent for the control of malaria (Tu, 2011). Patchouli, another sesquiterpene, is a popular aromatic found in colognes, perfumes and many other household cleaning products (Wu et al., 2006). Menthol is a monoterpene obtained from mint family plants and is a popular ingredient in many foods and consumer products (Bedoukian, 1983). Triterpenes such as squalene, obtained from various plant sources and the livers of deep sea sharks, have utility as a nutraceutical product, is used extensively in many types of cosmetics, has special utility as a lubricant for high performance machinery, and is a common adjuvant in many pharmaceutical formulations (Huang et al., 2009; Reddy and Couvreur, 2009; Bhilwade et al., 2010).
Terpenes are, however, generally made by plants and microbes in small amounts and components of complex mixtures that vary with growth and environmental conditions, making it difficult to reproducibly obtain large amounts of any one terpene constituent (Wu et al., 2006). Chemical synthesis of terpenes is often costly and inefficient (Nicolaou et al., 1994). Chemical synthesis also suffers from generating enantiomeric mixtures, which adds other complications if one particular stereochemical form of a terpene is desired. Given such difficulties, there are many on-going efforts to create robust, reliable and efficient biological systems for the production of distinct classes of terpenes, and more so for the generation of stereochemically pure forms of terpenes (Martin et al., 2003; Wu et al., 2006; Takahashi et al., 2007; Asadollahi et al., 2008; Kirby et al., 2008; Seki et al., 2008; Keasling, 2009; Asadollahi et al., 2010; Fischer et al., 2011).